Method of and apparatus for demagnetizing a magnetic material

ABSTRACT

To demagnetize a magnetic material, the material is positioned within a field of magnetic flux generated by an electrical induction coil having applied thereto a constant amplitude, alternating voltage wave, the half cycles of which alternate in polarity and successively and incrementally increase in frequency. The flux alternates in polarity with the polarity of the voltage wave, and the magnitude, or strength, of the flux is in accordance with the current flow through the coil, or is inversely in accordance with the impedance of the coil. As the frequency of the voltage wave incrementally increases from an initial low value to a higher value, the magnitude of the magnetic flux incrementally decreases from an initial high value to a lower value, and the alternating polarity, decreasing magnitude magnetic flux effectively demagnetizes the magnetic material. The invention finds utility in demagnetizing magnetic material cores of magnetic operating coils.

B11131.- mumiem cow United States Patent 1191 Maddox 1451 July 15, 1975METHOD OF AND APPARATUS FOR [57] ABSTRACT DEMAGNETIZING A MAGNETIC Todemagnetize a magnetic material, the material is MATERIAL positionedwithin a field of magnetic flux generated by [75] Inventor; Harry L.Maddox, Revnoldsburg an electrical induction coil having applied theretoa Ohio constant amplitude, alternating voltage wave, the half cycles ofwhich alternate in polarity and successively Asslgneei Western ElectricCompany, and incrementally increase in frequency. The flux al- 'p i NewYork ternates in polarity with the polarity of the voltage [22] Filed.Apt 29, 1974 wave, and the magnitude, or strength, of the flux is inaccordance with the current flow through the coil, or p N05 9. isinversely in accordance with the impedance of the coil. As the frequencyof the voltage wave incremen- [52] US. Cl. 317/1575 tally increases froman initial low value to a higher 51 1m. 01. 11011 13/00 value, themagnitude of the magnetic flux incremen- [58] Field of Search 317/1575tally decreases from an initial high value to a lower value, and thealternating polarity, decreasing magni- [56] References Cited tudemagnetic flux effectively demagnetizes the mag- UNITED STATES PATENTSnetic material. The invention finds utility in demagne- 2 962 560 M960 FI 17/ 57 5 tizing magnetic material cores of magnetic operating 0 se3,038,036 6/1962 Young et al. 317/1575 S 3,093.774 6/1963 Christiansonet a1. 317/1575 3,716,763 2/1973 Houze, Jr. et al. 317/1575 PrimaryExaminerL. T. Hix 11 Claims 3 Drawing Figures Attorney. Agent, or FirmR.A. Lloyd J- 44 84 7' 72 68 100 5 28 36 TOGGLE Ki *1 \FLIP-FLOP so 34 264 k E? 92 [INCREASING 96 FREQUENCY 2 3 osc 2/48 88 METHOD OF ANDAPPARATUS FOR DEMAGNETIZING A MAGNETIC MATERIAL BACKGROUND OF THEINVENTION 1. Field of 'the Invention The present invention relates to amethod of, and apparatus for, demagnetizing a magnetic material, and inparticular to a method of, and apparatus for, demagnetizing a magneticmaterial with a flux field generated by an electrical induction coilhaving an increasing frequency signal applied thereacross.

2. Description of the Prior Art Demagnetizing magnetic material is wellknown in the prior art, For example, in the manufacture of a productemploying a magnetic material, such as an electromagnetic coil having amagnetic material core, it is often necessary to demagnetize themagnetic material, prior to transport or storage of the coil, to preventthe attraction thereto of contaminants of a magnetic nature as a resultof residual magnetism therein. In the case of a magnetic coil having amagnetic material core, the conventional technique for demagnetizing thecore is to apply a constant frequency, sinusoidal signal across thecoil, and to then decrease, or modulate, the signal in amplitude untilthe current flow through the coil is essentially zero. With a constantfrequency signal applied across the coil, the impedance of the coilremains essentially constant, and the magnitude of the current flowtherethrough decreases as a result of the decreasing amplitude of thesignal. This generates within the core a magnetic field of flux whichalternates in polarity with the signal, and which decreases in magnitudewith the decrease in magnitude of the signal.

This conventional technique has several disadvantages. For example, if amechanical voltage regulation device, such as a variac, is employed todecrease the amplitude of a demagnetizing signal applied across thecoil, any noise or discontinuity in the decaying signal for the purposeof demagnetization can introduce, rather than eliminate, residualmagnetism in the magnetic coil. Furthermore, if an electronic circuit,as compared with a mechanical device such as a variac, is employed toeliminate noise in the demagnetizing signal, the output stage of thecircuit must initially conduct large currents when the amplitude of thedemagnetizing signal is large, and must later dissipate a large amountof power as the amplitude of the demagnetizing signal is decreased.Furthermore, the load impedance exhibited by a magnetic coil may shiftrapidly as the magnetic drive field exceeds the coercive force of thecore material thereof, and when this occurs the sudden change in currentthrough the coil may modulate the demagnetizing signal in such a way asto introduce residual magnetism into the core. In this case, if aplurality of parallel connected magnetic coils are simultaneouslydemagnetized, the current modulation caused by an individual one of thecoils, as the coercive force of the core material is exceeded by thedrive signal, introduces magnetism in all of the coils as a result ofthe altered drive signal. Also, as the strength of the magnetic field offlux must be changed in small increments to effectively demagnetize amagnetic material, the time required to demagnetize the material may beconsiderable since the minimum time between each incremental decrease isbased upon the period of a cycle of the constant frequency demagnetizingsignal.

SUMMARY OF THE INVENTION In accordance with the present invention, asystem for imparting a magnetic bias to a magnetic medium includescircuitry for generating a constant amplitude, alternating polaritysquare wave which increases in frequency from an initial first frequencyvalue to a second and higher frequency value, and an electricalinduction coil, energized by the square wave, for generating through themagnetic medium amagnetic field of flux having a polarity determined bythe polarity of the square wave and a strength which decreases from thefirst level to essentially zero as the impedance exhibited by the coilto the square wave increases as the frequency of the square waveincreases from the first to the second frequency value.

Preferably, an iscillator generates an initial signal the cycles ofwhich successively and incrementally increase in frequency andcircuitry, which includes a transistor the conduction of which iscontrolled by the application of the initial signal to the base thereof,and a diode in series with a Zener diode across the emitter-collector ofthe transistor, is responsive to the initial signal for generating asquare wave signal having a constant amplitude and a frequency which isdirectly in accordance with the frequency of the initial signal.Additional circuitry is connected to the emitter-collector of thetransistor and to the juncture between the diode and the Zener diode for(l) biasing the Zener diode into conduction through the diode and forholding the transistor in a nonconductive state during the applicationof the positive excursions of the initial signal to the base thereof togenerate across the emitter-collector thereof a first potential, and (2)biasing the transistor into conduction, and the Zener diode intononconduction through the diode, during the negative excursions of theinitial signal to generate across the emittercollector of the transistora second potential, whereby the alternating positive and negativeexcursions of the initial signal generate across the emitter-collectorof the transistor the square wave signal having the constant amplitudefrom the first to the second potential and a frequency which is directlyin accordance with the frequency of the initial signal.

The square wave signal is applied to a low-pass filter which produces atan output thereof an alternating polarity demagnetizing signal which hasa frequency equal to the frequency of the square wave signal and anamplitude which is essentially constant when the frequency of the squarewave signal is less than a predetermined value and which decreases whenthe frequency of the square wave signal is at least equal to thepredetermined frequency value. This demagnetizing signal is appliedacross the induction coil for generating through the magnetic medium,which is inductively coupled therewith, the magnetic field of fluxhaving a polarity which alternates with the polarity of thedemagnetizing signal and a strength which decreases from an initialvalue, when the square wave is initially applied thereacross, to a valuewhich is essentially zero when the frequency of the square wave signalis equal to the predetermined frequency, so that as the frequency of thesquare wave signal incrementally increases from an initial value to thepredetermined frequency value the strength of the field of magnetic fluxwithin the magnetic medium incrementally decreases from an initial valueto essentially zero to successively and incrementally decrease themagnitude of the magnetic flux within the magneticmedium to demagnetizethe mag netic medium.

One specific aspect of the invention contemplates demagnetizing amagnetic material core of an electromagnetic coil. In this case, aconstant amplitude voltage wave, having half cycles which alternate inpolarity and successively and incrementally increase in frequency from afirst frequency value to a second and higher frequency value is appliedacross the electromagnetic coil. This generates in the magnetic materialcore an alternating magnetic flux having a polarity in accordance withthe polarity of the alternating voltage wave'and having a magnitudeinversely in accordance with the frequency of the alternating voltagewave, the second frequency being of sufficient value to reduce themagnitude of the magnetic flux within the core to essentially zero todemagnetize the core, and the frequencies between the first and secondfrequency values increasing in frequency in a relation to successivelyand incrementally decrease the magnitude of the magnetic flux within thecore.

Other advantages and features of the invention will be apparent uponconsideration of the following detailed description when taken inconjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates, partially in blockdiagram form and partially in schematic form, circuitry fordemagnetizing a magnetic material in accordance with the presentinvention;

FIG. 2 shows a waveform of the signal provided at the output of theincreasing frequency oscillator of FIG. 1, and

FIG. 3 shows a waveform of the signal provided at the output of theflip-flop of FIG. 1 in response to the signal of FIG. 2 at the inputthereof.

DETAILED DESCRIPTION Referring to FIG. 1 of the drawings, there is shownone embodiment of a circuit which may advantageously be employed toimpart a magnetic bias to, or to demagnetize a magnetic material, or amagnetic medium, in accordance with the present invention. In thiscircuit, to demagnetize a magnetic material, an increasing frequencyoscillator 12 applies to an electrical induction coil, throughintermediate circuitry, a constant amplitude alternating polaritydemagnetizing signal, or voltage wave, the successive cycles of whichare frequency modulated to incrementally increase in frequency from aninitial low first value to a second and higher value. This generates afield of magnetic flux within the coil, having a polarity whichalternates with the alternating polarity of the signal, and having amagnitude inversely in accordance with the frequency of the signal, todemagnetize a magnetic material placed within the field. For the purposeof describing the invention, the magnetic material may be a core 16, andthe induction coil may be an inductively wound winding around the core16, which together form an electromagnetic coil 24. However, it iswithin the contemplation of the invention that other magnetic mediumsmay be demagnetized, or have a magnetic bias imparted thereto, by beingplaced within a field of magnetic flux generated by, or by beinginductively coupled with, other types of induction coils, or magneticflux generation devices, to which a signal of the type generated by thecircuit is applied, and reference to a core and a winding of anelectromagnetic coil is intended to be illustrative only, and notlimiting.

The invention contemplates taking advantage of the increasing impedanceexhibited by an induction coil when an increasing frequency signal, orvoltage wave, is applied thereacross. In practice, an initially lowfrequency, constant amplitude signal is applied across the coil 20, andis then increased in frequency while maintaining the amplitude thereofconstant. While the frequency of the signal is low, the coil 20 exhibitsa relatively low impedance which results in a relatively largealternating current flow therethrough, and therefore in a correspondingrelatively large amplitude alternating shift in the field of magneticflux generated thereby. This field of magnetic flux passes through thecore 16 to magnetically switch the material thereof to impart a magneticbias thereto and, as will be seen, results in the demagnetization of thecore upon an increase in frequency of the signal.

As the frequency of the signal increases the impedance of the coil 20similarly increases, resulting in a decreasing magnitude alternatingcurrent flow therethrough, and therefore in corresponding decreasingamplitude shifts in the magnetic flux through the core 16. This occurssince the impedance of an induction coil is directly proportional to thefrequency of a signal applied thereacross, and for a constant amplitudesignal applied across a coil the current through the coil is inverselyproportional to the frequency of the signal.

To achieve optimum demagnetization of the core 16, the frequency of thedemagnetizing signal applied to the coil 20 is increased incrementallywith each cycle, or with each half cycle, of the signal, until thecurrent flow through the coil, as limited by the impedance of the coil,is essentially zero. This generates through the magnetic material core16 successive cycles, or excursions, of magnetic flux which alternate inpolarity, and which successively decrease in amplitude from a relativelyhigh initial value to a final value which is essentially zero, toeffectively demagnetize the core.

In the demagnetization of the core 16, it is necessary thatthe magnitudeof the alternating current through the coil 20, and therefore themagnitude of the magnetic flux excursions which magnetically switch themagnetic material core 16, be incrementally decreased from an initialvalue to zero. In a conventional demagnetizer where the amplitude, butnot the frequency, of the demagnetizing signal is varied, this isaccomplished by incrementally decreasing the amplitude of the constantfrequency signal from an initial value to zero. In the present inventionthis is accomplished by incrementally increasing the frequency of aconstant amplitude demagnetizing signal from an initial relatively lowvalue to a relatively high value. To minimize the time required todemagnetize the core 16, the frequency of the demagnetizing signal maybe increased at an exponential rate, which is possible since it is theincremental decrease in the magnitude of the half cycles of thealternating current through the coil 20 which is of concern, and not thetime interval between the incremental decreases. This, of course, allowsdemagnetization of the core 16 in a shorter time interval than would beattainable with a conventional demagnetizer where, to obtain a uniformincremental decrease in current through the coil with a constantfrequency signal applied thereto, it

is necessary to linearly decrease the amplitude of the signal appliedthereacross at a rate which is dependent upon the period of the cyclesof the signal.

More particularly, the increasing frequency oscillator 12 may be of thetype disclosed in my copending application Ser. No. 461,082, filed Apr.15, 1974, or may be comprised of a conventional wide range voltagecontrolled oscillator circuit which receives as an input the output froma conventional ramp voltage generator circuit. The oscillator 12generates at an output 28 thereof a signal 32, as shown in FIG. 2, theindividual cycles of which have a constant voltage excursion, oramplitude, and the successive cycles of which incrementally increase infrequency from an initial frequency value to a higher frequency value.In other words, successive cycles of the signal 32 have decreasingperiods, and the period I of the Nth cycle of the signal 32 is less thanthe period tof the preceding- Nthl cycle, and is greater than the periodtof the succeeding Nth+1 cycle.

The signal 32 at the output 28 of the oscillator 12 is applied through acapacitor 34 to a flip-flop 36 which receives a first operatingpotential from a source of reference potential 40, and a secondoperating potential from the juncture between a Zener diode 55 and aresistor 48 which are connected in series between the source ofreference potential 40 and a source of negative potential 52. The Zenerdiode 44 provides a regulated power source for flip-flop 36, and theoutput of the flip-flop, with the signal 32 at the input thereof, is asquare wave signal 56, as shown in FIG. 3, having alternating polarityhalf cycles which occur in response to each negative going transition 60of each cycle of the signal 32. In other words, the flip-flop 36 dividesthe signal 32 by two, and the output of the flip-flop alternatelychanges polarity with each cycle of the signal 32 at the negative goingtransition portion 60 thereof. This generates the signal 56 which is analternating voltage signal, or square wave, having alternating polarityhalf cycles which successively and incrementally increase in frequencyin accordance with the successive and incremental increase in frequencyof each successive cycle of the signal 32, and each successive halfcycle of the signal 56 has a period which is identical with the periodof a corresponding full cycle of the signal 32.

The signal 56 is applied through a resistor 64 to the base of atransistor 68 for controlling the conduction thereof. The transistor 68is connected at its base to the source of reference potential 40 througha resistor 72, the emitter of the transistor is connected directly tothe source of reference potential 40, and the collector thereof isconnected to the source of negative potential 52 through a resistor 76.

A diode 80 is connected in series with a Zener diode 84 between thesource of reference potential 40 and the source of negative potential 52through the resistor 76, and is connected at its cathode to thecollector of the transistor 68. A resistor 88 is connected between theanode of the diode 80 and the source of negative potential 52. Duringthe positive half cycles of the signal 56 the base of the transistor 68is positive with respect to the emitter thereof, the transistor 68 isnonconductive, and the Zener diode 84 conducts current from the sourceof reference potential 40 through the diode 80 and the resistor 76 tothe source of negative potential 52, as well as through the resistor 88to the source of negative potential. At this time, the potential at thecollector of the transistor 68 is essentially equal to the potential atthe anode of the diode 80. That is, the potential at the collector ofthe transistor 68 is equal to the relatively negative potential at theanode of the diode 80, as determined by the breakdown voltage of theZener diode 84, less the forward voltage drop of the diode 80.

During the negative half cycles of the signal 56 the base of thetransistor 68 is negative with respect to the emitter thereof, thetransistor 68 is conductive, and the potential at the collector thereofis essentially equal in value to the source of reference potential 40.At this time the diode 80 is reversed biased and does not conductcurrent therethrough as a result of a potential at its anode, as theZener diode 84 continues to conduct through the resistor 88, which isnegative with respect to the potential at its cathode. It is to beappreciated that the resistor 88 provides a conductive path for theZener diode 84 to maintain the Zener diode conductive, during the timewhen the transistor 68 conducts, when the Zener diode would otherwidebecome nonconductive as the potential at the collector of the transistor68 becomes essentially equal to the source of reference potential 40.This eliminates delay, as a result of the stabilizing time of the Zenerdiode 84, in reaching the potential to be applied to the collector ofthe transistor 68 when the transistor becomes nonconductive, andgenerates at the collector of the transistor 68 a well defined squarewave having symmetry in amplitude as well as symmetry in time, minuspredetermined incremental decrements, between successive half cyclesthereof.

The signal at the collector of the transistor 68 is applied through alow-pass filter, comprising a pair of resistors 92 and 96 and a pair ofcapacitors 100 and 104, to a power amplifier circuit 108 over aconductor 112. The output from the amplifier 108 is applied as ademagnetizing signal to the coil 20 of the electromagnetic coil 24, andthe values of the resistors 92 and 96 and the capacitors 100 and 104 ofthe low-pass filter are such that the signal on the conductor 112, asapplied to the coil 20 through the amplifier 108, has a constantamplitude until the frequency thereof becomes sufficiently high that theimpedance of the coil 20 becomes sufficiently great that essentially nocurrent flows therethrough, and thereafter is reduced in amplitude asthe frequency thereof continues to increase. That is, as the signal atthe collector of the transistor 68, as applied over the conductor 112through the low-pass filter, increases in frequency from an initialfirst relatively low frequency value to a second and higher frequencyvalue, at which point essentially no current flows through the coil 20,the lowpass filter provides a constant amplitude signal on the conductor112 until the frequency of the signal at the collector of the transistor68 reaches the second frequency value, and thereafter modulates, .ordecreases, the amplitude of the signal on the conductor 1 12 by anamount directly in accordance with the frequency value thereof. Thiseffectively and automatically terminates the application of thedemagnetizing signal to the coil 20.

The demagnetizing signal, or voltage wave, at the output of theamplifier 108 applied across the coil 20 has a waveform essentially asshown in FIG. 3. That is, the demagnetizing signal is a constantamplitude voltage wave having alternating polarity half cycles whichsuccessively and incrementally increase in frequency.

This signal generates a field of magnetic flux, with the coil 20, whichpasses through the magnetic material core 16 for imparting a magneticbias thereto and which has a polarity determined by the polarity of thesignal and a magnitude, or strength, which varies in versely inaccordance with the frequency of the'signal. Therefore, as eachsuccessive half cycle of the demagnetizing signal incrementallyincreases in frequency the impedance of the coil 20 incrementallyincreases in value, the current through the coil 20 incrementallydecreases in value, and the magnitude of the magnetic flux generated bythe coil 20 through the core 16 incrementally decreases in value. Thisimparts a magnetic bias to the core 16 and results in an incrementlstep-bystep demagnetization of the core 16 with each successive halfcycle of the demagnetizing signal as the frequency thereof incrementallyincreases in value with respect to the frequency of the preceding halfcycle.

When the frequency of the demagnetizing signal increases to a valuewhere the impedance exhibited by the coil 20 is sufficiently great sothat essentially no current flows through the coil 20 the magnitude, orstrength, of the magnetic field of flux passing through the magneticmaterial core 16 is essentailly zero, and the core 16 is demagnetized.At or above this fre quency, to eliminate the possibility of introducingresidual magnetism into the core 16 upon the termination of thedemagnetizing signal applied to the coil 20, the low-pass filtergradually reduces the amplitude of the signal with continued andincremental increases in the frequency thereof to effectively remove thedemagnetizing signal from the coil 20 without a sudden change in valuethereof. It should be noted that demagnetization of the core 16 isaccomplished as a result of the increasing frequency of thedemagnetizing signal prior to the time that the amplitude of the signalis reduced by the low-pass filter, and that at the time the low-passfilter becomes operative to reduce the amplitude of the demagnetizingsignal demagnetization of the core 16 has been completed.

While one embodiment of the invention has been described in detail, itis understood that various other modifications and embodiments may bedevised by one skilled in the art without departing from the spirit andscope of the invention. For example, while the demagnetizing signal hasbeen described as having half cycles which successively andincrementally increase in frequency, it is within the contemplation ofthe invention to sequentially increase the frequency of a demagnetizingsignal upon each occurrence of a group of half cycles or full cycles.Furthermore, while the invention has been described with respect to thedemagnetization of a magnetic material core of an electromagnetic coil,it is within the contemplation of the invention that any induction coilcapable of generating a field of magnetic flux, or any other type ofdevice which is capable of being energized by an alternating voltagesignal for imparting a desired magnetic bias to a magnetic materialwhich varies in strength in accordance with frequency of the signal, maybe employed to demagnetize a magnetic material other than a core of anelectromagnetic coil, or any other magnetic medium.

What is claimed is: is essentially 1. In an apparatus for demagnetizinga magnetic material core of an electromagnetic coil:

means for generating a constant amplitude voltage wave, having halfcycles which alternate in polarity and successively and incrementallyincrease in frequency from a first frequency value to a second andhigher frequency value, to be applied across the electromagnetic coil togenerate in the magnetic material core thereof an alternating magneticflux having a polarity in accordance with the polarity of the voltagewave and having a magnitude inversely in accordance with the frequencyof the voltage wave, the second frequency being of sufficient value toreduce the magnitude of the magnetic flux within the core to essentiallyzero, and the frequencies between the first and the second valuesincrementally increasing in frequency in a relation to successively andincrementally decrease the magnitude of the magnetic flux within thecore, to demagnetize the core, and

means for applying the voltage wave across the electromagnetic coilwhile the frequency thereof increases from the first to the secondfrequency value to demagnetize the core.

2. In an apparatus as set forth in claim 1, further including:

means, operative upon the frequency of the alternating voltage wavereaching and exceeding the second frequency value, for decreasing theamplitude of successive cycles of the voltage wave directly inaccordance with the frequency thereof.

3. In a system for imparting a magnetic bias to a magnetic medium:

means for generating a constant amplitude, alternating polarity squarewave which increases in frequency from an initial first frequency valueto a second and higher frequency value, and

means, energizable by the square wave, for imparting to the magneticmedium a magnetic bias having a polarity determined by the polarity ofthe square wave and a strength which decreases from a first level whenthe frequency of the wave is at the first frequency value to a secondlevel which is essentially Zero as the frequency of the wave increasesto the second frequency value.

4. In a system as set forth in claim 3, wherein the means for impartinga magnetic bias to the magnetic medium includes:

an electrical induction coil, energized by the square wave, forgenerating through the magnetic medium a magnetic field of flux having apolarity determined by the polarity of the square wave and a strengthwhich decreases from the first level to essentially Zero as theimpedance exhibited by the coil to the square wave increases in value asthe frequency of the square wave increases from the first to the secondfrequency value.

5. In an apparatus for demagnetizing a magnetic medium:

means for generating a frequency signal, the cycles of whichsuccessively and incrementally increase in frequency;

means, responsive to the increasing frequency signal,

for producing a constant amplitude, alternating polarity voltage wavehaving a frequency in accordance with the frequency of the increasingfrequency signal;

a low-pass filter, responsive to the voltage wave at an input thereoffor producing at an output thereof an alternataing polaritydemagnetizing signal having a frequency equal to the frequency of thevoltage wave and having an amplitude which is essentially constant whenthe frequency of the voltage wave is less than a predetermined frequencyvalue and which decreases when the frequency of the voltage wave isgreater than the predetermined frequency value; i

an electrical induction coil for generating a field of magnetic fluxthrough the magnetic medium, and

means for applying the demagnetizing signal across the induction coilfor generating the'field of mag quency, whereby as the frequency of thedemagnetizing signal increases from an initial value to thepredetermined value, the strength of the field of flux decreases from aninitial value to essentially zero to demagnetize the magnetic medium.

'netic material inductively coupled therewith a magnetic field of fluxhaving a polarity which alternates with the polarity of thedemagnetizing signal and a strength which decreases from an initialvalue when the square wave signal is initially applied thereacross to avalue which is essentially Zero when the frequency of the square wavesignal "is equal to the predetermined frequency, whereby as thefrequency of the square wave signal incrementally increases from aninitial value to the predetermined frequency value the strength of thefield of'magnetic flux within the magnetic material incrementallydecreases from an initial value to essentially zero to successively andincrementally decrease the magnitude of the magnetic flux within themagnetic material to demagnetize the core.

7. In a method of demagnetizing a magnetic material core of anelectromagnetic coil:

generating a constant amplitude voltage wave having half cycles whichalternate in polarity and successively and incrementally increase infrequency from a first frequency value to a second and higher 6. In asystem for demagnetizing a magnetic material which is inductivelycoupled to an electrical induction coil:

oscillator means for generating an initial signal the frequency value,the second frequency value being sufficient, when the voltage wave isapplied across the electromagnetic coil, to increase the impedcycles ofwhich successively and incrementally increase in frequency;

means, responsive to the initial signal, for generating diode intoconduction through the diode and for ance exhibited thereby sufficientlyto reduce the current flow therethrough to essentially zero, and

applying the voltage wave across the electromagnetic coil to generate inthe magnetic material core c u an a with the frequency of the initialsignal, the means i m accordance wlth polamy i the including atransistor the conduction of which is i voltage wave having a magmtudemversely controlled by the application of the initial signal to maccordance with h frequency of the vpltage the base thereof, and a diodein series with a Zener wave and therefore dniectly m accordance wlth thediode across the emitter-collector of the transistor; current throughthe 9 whereby as the frequency means, connected to the emitter-collectorof the of the voltage waive Increases to the secorld fretransistor andto the juncture between the diode quencfy the magmtude of the field mthe and the Zener diode for (1) biasing the Zener core incrementallydecreases from an initial value to essentially zero to demagnetlze thecore.

8. In a method as set forth in claim 7, further including:

decreasing the amplitude of the voltage wave when holding the transistorin a nonconductive state during the application of the positiveexcursions of the initial signal to the base thereof to generate acrossa low-pass filter, responsive to the square wave signal at an inputthereof for producing at an output thereof an alternating polaritydemagnetizing signal having a frequency equal to the frequency of thesquare wave signal and having an amplitude which is essentially constantwhen the frequency of the square wave signal is less than apredetermined frequency value and which decreases when the frequency ofthe square wave signal is at least equal to the predetermined frequencyvalue, and

means for applying the demagnetizing signal across the induction coilfor generating through the magthe frequency thereof reaches and exceedsthe second frequency.

9. In a method of demagnetizing a magnetic medium: generating a constantamplitude alternating polarity square wave which increases in frequencyfrom an initial first frequency value to a second and higher frequencyvalue;

applying the square wave across an electrical induction coil forgenerating therewith a field of magnetic flux having a directiondetermined by the polarity of the square wave and having a magnitudewhich varies inversely in accordance with the frequency of the squarewave, the second frequency value of the square wave being sufficientthat the magnitude of the field of flux generated by the induction coilis essentially zero, and

positioning the magnetic medium within the generated field of magneticflux, whereby as the frequency of the square wave increases from theinitial first frequency value to the second and higher frequency value,the magnitude of the magnetic flux decreases from an initial value to avalue which is essentially zero to demagnetize the magnetic medium.

10. In a method as set forth in claim 9, further includdecreasing theamplitude of successive cycles of the square wave when the frequencythereof reaches and exceeds the second frequency value. 11. In a methodofdemagnetizing a magnetic medium:

generating a frequency signal, the cycles of which successively andincrementally increase in frequency; generating a constant amplitude,alternating polarity voltage wave having a frequency determined by thefrequency of the increasing frequency signal; decreasing the amplitudeof cycles of the voltage wave which have a frequency at least equal to apredetermined frequency, and

applying the voltage wave across an electrical inducpositioning themagnetic medium within the generated field of flux, whereby asthefrequency of the voltage wave increases from an initial value to thepredetermined value, the strength of the field of flux decreases from aninitial value to essentially zero to demagnetize the magnetic medium.

1. In an apparatus for demagnetizing a magnetic material core of anelectromagnetic coil: means for generating a constant amplitude voltagewave, having half cycles which alternate in polarity and successivelyand incrementally increase in frequency from a first frequency value toa second and higher frequency value, to be applied across theelectromagnetic coil to generate in the magnetic material core thereofan alternating magnetic flux having a polarity in accordance with thepolarity of the voltage wave and having a magnitude inversely inaccordance with the frequency of the voltage wave, the second frequencybeing of sufficient value to reduce the magnitude of the magnetic fluxwithin the core to essentially zero, and the frequencies between thefirst and the second values incrementally increasing in frequency in arelation to successively and incrementally decrease the magnitude of themagnetic flux within the core, to demagnetize the core, and means forapplying the voltage wave across the electromagnetic coil while thefrequency thereof increases from the first to the second frequency valueto demagnetize the core.
 2. In an apparatus as set forth in claim 1,further including: means, operative upon the frequency of thealternating voltage wave reaching and exceeding the second frequencyvalue, for decreasing the amplitude of successive cycles of the voltagewave directly in accordance with the frequency thereof.
 3. In a systemfor imparting a magnetic bias to a magnetic medium: means for generatinga constant amplitude, alternating polarity square wave which increasesin frequency from an initial first frequency value to a second andhigher frequency value, and means, energizable by the square wave, forimparting to the magnetic medium a magnetic bias having a polaritydetermined by the polarity of the square wave and a strength whichdecreases from a first level when the frequency of the wave is at thefirst frequency value to a second level which is essentially zero as thefrequency of the wave increases to the second frequency value.
 4. In asystem as set forth in claim 3, wherein the means for imparting amagnetic bias to the magnetic medium includes: an electrical inductioncoil, energized by the square wave, for generating through the magneticmedium a magnetic field of flux having a polarity determined by thepolarity of the square wave and a strength which decreases from thefirst level to essentially zero as the impedance exhibited by the coilto the square wave increases in value as the frequency of the squarewave increases from the first to the second frequency value.
 5. In anapparatus for demagnetizing a magnetic medium: means for generating afrequency signal, the cycles of which successively and incrementallyincrease in frequency; means, responsive to the increasing frequencysignal, for producing a constant amplitude, alternating polarity voltagewave having a frequency in accordance with the frequency of theincreasing frequency signal; a low-pass filter, responsive to thevoltage wave at an input thereof for producing at an output thereof analternataing polarity demagnetizing signal having a frequency equal tothe frequency of the voltage wave and having an amplitude which isessentially constant when the frequency of the voltage wave is less thana predetermined frequency value and which decreases when the frequencyof the voltage wave is greater than the predetermined frequency value;an electrical induction coil for generating a field of magnetic fluxthrough the magnetic medium, and means for applying the demagnetizingsignal across the induction coil for generating the field of magneticflux through the magnetic medium, the polarity of the flux being inaccordance with the polarity of the demagnetizing signal and thestrength of the flux being inversely in accordance with the frequency ofthe demagnetizing signal, the strength of the field of flux beingessentially zero when the demagnetizing signal is at the predeterminedfrequency, whereby as the frequency of the demagnetizing signalincreases from an initial value to the predetermined value, the strengthof the field of flux decreases from an initial value to essentially zeroto demagnetize the magnetic medium.
 6. In a system for demagnetizing amagnetic material which is inductively coupled to an electricalinduction coil: oscillator means for generating an initial signal thecycles of which successively and incrementally increase in frequency;means, responsive to the initial signal, for generating a square wavesignal having a constant amplitude and a frequency which is directly inaccordance with the frequency of the initial signal, the means includinga transistor the conduction of which is controlled by the application ofthe initial signal to the base thereof, and a diode in series with aZener diode across the emitter-collector of the transistor; means,connected to the emitter-collector of the transistor and to the juncturebetween the diode and the Zener diode for (1) biasing the Zener diodeinto conduction through the diode and for holding the transistor in anonconductive state during the application of the positive excursions ofthe initial signal to the base thereof to generate across theemitter-collector thereof a first potential, and (2) biasing thetransistor into conduction, and the Zener diode into nonconductionthrough the diode, during the negative excursions of the initial signalto generate across the emitter-collector of the transistor a secondpotential, whereby the alternating positive and negative excursions ofthe initial signal generate across the emitter-collector of thetransistor the square wave signal having the constant amplitude from thefirst to the second potential and a frequency directly in accordancewith the frequency of the initial signal; a low-pass filter, responsiveto the square wave signal at an input thereof for producing at an outputthereof an alternating polarity demagnetizing signal having a frequencyequal to the frequency of the square wave signal and having an amplitudewhich is essentially constant when the frequency of the square wavesignal is less than a predetermined frequency value and which decreaseswhen the frequency of the square wave signal is at least equal to thepredetermined frequency value, and means for applying the demagnetizingsignal across the induction coil for generating through the magneticmaterial inductively coupled therewith a magnetic field of flux having apolarity which alternates with the polarity of the demagnetizing signaland a strength which decreases from an initial value when the squarewave signal is initially applied thereacross to a value which isessentially zero when the frequency of the square wave signal is equalto the predetermined frequency, whereby as the frequency of the squarewave signal incrementally increases from an initial value to thepredetermined frequency value the strength of the field of magnetic fluxwithin the magnetic material incrementally decreases from an initialvalue to essentially zero to successively and incrementally decrease themagnitude of the magnetic flux within the magnetic material todemagnetize the core.
 7. In a method of demagnetizing a magneticmaterial core of an electromagnetic coil: generating a constantamplitude voltage wave having half cycles which alternate in polarityand successively and incrementally increase in frequency from a firstfrequency value to a second and higher frequency value, the secondfrequency value being sufficient, when the voltage wave is appliedacross the electromagnetic coil, to increase the impedance exhibitedthereby sufficiently to reduce the current flow therethrough toessentially zero, and applying the voltage wave across theelectromagnetic coil to generate in the magnetic material core thereofan alternating magnetic flux having a polarity in accordance with thepolarity of the alternating voltage wave and having a magnitudeinversely in accordance with the frequency of the voltage wave, andtherefore directly in accordance with the current through the coil,whereby as the frequency of the voltage wave increases to the secondfrequency the magnitude of the field of flux in the core incrementallydecreases from an initial value to essentially zero to demagnetize thecore.
 8. In a method as set forth in claim 7, further including:decreasing the amplitude of the voltage wave when the frequency thereofreaches and exceeds the second frequency.
 9. In a method ofdemagnetizing a magnetic medium: generating a constant amplitudealternating polarity square wave which increases in frequency from aninitial first frequency value to a second and higher frequency value;applying the square wave across an electrical induction coil forgenerating therewith a field of magnetic flux having a directiondetermined by the polarity of the square wave and having a magnitudewhich varies inversely in accordance with the frequency of the squarewave, the second frequency value of the square wave being sufficientthat the magnitude of the field of flux generated by the induction coilis essentially zero, and positioning the magnetic medium within thegenerated field of magnetic flux, whereby as the frequency of the squarewave increases from the initial first frequency value to the second andhigher frequency value, the magnitude of the magnetic flux decreasesfrom an initial value to a value which is essentially zero todemagnetize the magnetic medium.
 10. In a method as set forth in claim9, further including: decreasing the amplitude of successive cycles ofthe square wave when the frequency thereof reaches and exceeds thesecond frequency value.
 11. In a method of demagnetizing a magneticmedium: generating a frequency signal, the cycles of which successivelyand incrementally increase in frequency; generating a constantamplitude, alternating polarity voltage wave having a frequencydetermined by the frequency of the increasing frequency signal;decreasing the amplitude of cycles of the voltage wave which have afreqUency at least equal to a predetermined frequency, and applying thevoltage wave across an electrical induction coil to generate with thecoil a field of magnetic flux having a polarity in accordance with thepolarity of the voltage wave and a strength inversely in accordance withthe frequency of the wave, the strength of the field of flux beingessentially zero when the voltage wave is at the predeterminedfrequency, and positioning the magnetic medium within the generatedfield of flux, whereby as the frequency of the voltage wave increasesfrom an initial value to the predetermined value, the strength of thefield of flux decreases from an initial value to essentially zero todemagnetize the magnetic medium.